1. Field of the Invention
This invention relates to an electrode assembly used in the production of metal in electrolytic reduction cells. More particularly, this invention relates to improvements in current distribution through the electrode assembly to reduce the voltage drop therein and improve the heat dissipation.
2. Description of the Prior Art
In the production of metal, such as aluminum, in an electrolytic reduction cell, anodes and cathodes are used which are constructed, principally, of conductive material, such as carbon, which will conduct the high currents used for the electrolytic reduction to the molten salt bath in the cell. Carbon electrodes are normally used to avoid contamination of the bath with foreign metals and to lower necessary reduction voltage.
The current is normally carried to the electrode by large conductor busses which, in the case of the anode is, in turn, directly connected to the anode via a metal rod which also functions as a mechanical support for the anode and is lowered or raised in the cell and incidentally as a cooling heat sink. The need for the anode to function as a heat sink varies as cell current density changes.
Conventionally, the anode is attached to the metallic rod by inserting the rod into a central bore formed in the top of the anode. An electrically conducting ram mix may then be placed into the space between the rod and the bore in the anode. This connection, however, can be less than satisfactory from a mechanical standpoint, speed of assembly and electrically as well as by providing a higher resistance at the interface. This problem has been partially addressed in the prior art. For example, German Pat. No. 1,187,807 discloses a carbon anode having one or more cavities to receive a metal stub or rod. The surfaces of the cavities have grooves or teeth to increase the surface area which is said to provide better conductivity of the current from the rod into the anode.
German Pat. No. 1,937,411 provides for a cast iron structure to be poured around a steel stub placed in the end of a carbon anode. The purpose of the cast iron structure apparently, is to spread the current distribution across the top surface of the anode, as well as to lock the metal rod or stub to the anode by providing an undercutting in the sidewall of the recess cut into the top surface of the anode to receive the molten cast iron. The cast iron, as it solidifies, then provides a dovetail-like fit in the anode to prevent or inhibit the stub from separating from the anode.
While such arrangements do provide better mechanical bonding between the steel support rod and the anode and do provide some current distribution improvements, the current distribution is still limited to an area or volume immediately surrounding the metal rod or, at best, only across the upper surface of the anode.
Russian Pat. No. 378,524 illustrates a carbon electrode structure having the usual central bore to receive a metal stub and also having a series of holes drilled into the carbon block parallel to the central bore to receive cast iron rods. Openings are then cut into the carbon between the central bore and the cast iron rods to permit cast iron bridge pieces to be poured to connect the cast iron rods to the metal stub. The purpose of the rods is to reduce power losses.
Despite these attempts to distribute the current more evenly in the anode, there remains a need to optimize the distribution of current from the central stub to the anode as well as from the rod within the cathode to reduce voltage drops therebetween as well as to dissipate heat generated by such voltage drops which can otherwise result in burnoffs of the anodes.